The fluid can be an electrically conductive melt which flows through a channel from a metallurgical vessel. The components are then typically gases or slag. Coherent components shall be understood as being regions of components which expand especially in the direction of flow such as threads and whose extension in the direction of flow is typically much larger than the channel diameter. Discrete components shall be understood as incoherent regions of components or particles whose extension in the direction of motion is typically smaller than the channel diameter.
The invention similarly relates to a method in which the disturbance of an electromagnetic field which penetrates the flowing melt at least partly is evaluated at a measuring point which is flowed through by the melt and which is generated by at least one transmitter coil which is flowed through by alternating current. Generic apparatuses comprise in addition to the transmitter coil a measuring element for measuring disturbances of the field in the measuring point and an evaluating device by means of which non-metallic components such as gases or slag are detectable by means of a disturbance in the field.
When re-filling or pouring metal melt from a metallurgical vessel (e.g. a converter, kettle or header), it is desirable not to transfer slag (non-metallic phase components) swimming on the metal surface to the next vessel. The generic methods and apparatuses are used for monitoring the outflowing melt, so that measures can be taken during the detection of slag in order to suppress the transfer of slag. These measures can consist of the generation of a warning signal or the automatic termination of the re-filling process or taking an influence on the flow. Measures for influencing the flow are the reduction of the outflow cross section or the injection of gases for example, typically argon or nitrogen, into the outflow region in order to prevent the formation of eddies. The measuring point is typically arranged above a control member controlling the outflow.
In the generic methods and apparatuses an alternating magnetic field is established at a measuring point in the pouring channel by means of a transmitter coil which is flowed through by alternating current. This field produces a voltage in the flowing melt which on its part produces an eddy current in the electrically conductive melt. This current on its part produces an alternating magnetic field which can be measured with a measuring element.
If the outflowing melt contains components which show a lower electric conductivity than the metal, the current distribution in the melt changes and thus the field strength of the alternating magnetic field. By measuring the change of the magnetic field strength at the measuring point, entrained non-metallic components are detected. If the summarized change of the field strength reaches a threshold amplitude, a warning and/or control signal is triggered.
A generic method and a respective apparatus has been described in DE 31 42 681 A1. It has been proposed here to measure changes in the electromagnetic field at the measuring point by means of the voltage induced in a receiver coil, which like the transmitter coil is arranged concentrically about the pouring channel of a metallurgical vessel.
Improvements of this method are shown in DE 34 39 369 A1 and DE 37 22 795 A1. It is proposed on the one hand to charge the transmitter coil with different, mutually overlapping frequencies. A highly differentiated picture of the flowing melt is recognizable from the reaction in the receiver coil, so that already a very low share of slag can be detected in the same. It is proposed on the other hand to arrange the transmitter and receiver coils in a non-magnetic vessel in order to avoid the signal drift (i.e. the distortion of the magnetic field strength as measured at the measuring point) due to the temperature changes of the ferromagnetic floor plate of a metallurgical vessel.
It is also generally known to compensate the signal drift by measuring the coil temperature and correcting the measured values at least in part. If ferromagnetic metal parts are located close to the coils, the interconnection between temperature and drift of the measuring signals is non-linear, so that the influence of the temperature on the signals cannot be eliminated completely. Despite improvements already achieved, a residual quantity of slag is also regularly transferred to the next vessel with the known generic methods and apparatuses towards the end of the discharging process. Rising requirements placed on the degree of purity of the final product can therefore frequently not be fulfilled with the known methods and apparatuses.
The reason for this low residual quantity is on the one hand the technically caused detection limit of the employed generic apparatuses and on the other hand a process which is known as “continuous intermixing”: Towards the end of the discharging process a so-called whirl sink can occur during the outflow of a liquid from a vessel. The slag “swimming” on the surface of the metallic melt as a result of its lower density is drawn into the pouring channel as a “thread” by such a whirl, whereby its cross section and thus the percentage by mass of the slag in the melt rises continuously from virtually zero. As long as the percentage by mass of the slag in the melt lies below the threshold amplitude of the generic apparatus, it will not be detected by the same and the slag will continue to flow along in an undetected fashion. The threshold amplitude of the generic apparatuses cannot be reduced at will because disturbance signals which are generally known in signal engineering as “noise” and especially temperature drifts will superpose the measuring signals. As a result of these unavoidable error sources a quasi “natural” detection limit is defined for the generic methods and apparatuses which cannot be undercut.
According to standing doctrine, the known process of “continuous intermixing” of slag during the occurrence of a whirl sink is characterized in that the slag is entrained from the start of the intermixing process in form of a “slag thread” whose cross section rises more or less continuously. In the spectral analysis of measured values of the known generic apparatuses pulse-like disturbances of the field were observed at the measuring point. The form of the pulses of these disturbances corresponds to that of discrete, electrically non-conductive concentrations in the melt which pass through the electromagnetic field of the transmitter coil.
As a result of a purposeful observation of these disturbances which according to standing doctrine are negligible as “noise” it was proven that the continuous sucking in of a thin thread frequently precedes the sucking in of smaller discrete quantities or that the thread is also interrupted several times.